RTTY
Teleprinting over radio — the teleprinter's characters encoded as frequency-shifting tones, the backbone of weather reports, news wires and military traffic since the 1920s.
RTTY (radioteletype) sends the Baudot/ITA2 code as frequency-shift keying (FSK): two tones, mark and space, often 170 Hz apart, at the standard rate of 45.45 baud (about 60 words per minute). At the far end a teleprinter such as the Teletype Model 15 or 28 printed the received text onto tape or paper.
From the 1920s and 30s RTTY carried news-agency traffic (press RTTY), diplomatic and military nets, and radio amateurs. It was gradually displaced by more efficient digital modes like PSK31, but survives as an active ham mode — one of the few places Baudot code still goes out over the air.